The present invention relates to a dose counter for a portable dispensing device of the type used for dispensing discrete amounts of a fluid or particulate material entrained in an air or other propellant stream. In particular, the invention is concerned with dose counters for portable dispensing devices of the metered dose inhaler type which are well known in the art of medicine for treatment of, or alleviation of the effects of, respiratory complaints such as asthma.
Metered dose inhalers typically consist of a medicament-containing vessel and an actuator body having a drug delivery outlet. The present invention will find utility in any type of metered dose inhaler in which the contents of the medicament reservoir are invisible to the user, including pressurised metered dose inhalers (of both manually-operable and breath-actuated types), dry powder inhalers, or such like.
The medicament-containing vessel may be a pressurised canister containing a mixture of active drug and propellant. Such canisters are usually formed from a deep drawn aluminium cup portion having a crimped lid portion which carries a metering valve assembly. The metering valve assembly is provided with a protruding valve stem which, in use, is inserted as a tight push fit into a so-called xe2x80x9cstem blockxe2x80x9d in the actuator body.
To actuate the conventional manually-operable inhaler, the user applies a compressive force to the closed end of the canister. The internal components of the metering valve assembly are spring loaded so that, typically, a compressive force of between 15 and 30 N is required to activate the device. In response to this compressive force, the canister moves axially with respect to the valve stem by an amount varying between about 2 and 4 mm. This degree of axial movement is sufficient to actuate the metering valve and cause a metered quantity of the drug and propellant to be expelled through the valve stem. This is then released into the mouthpiece via a nozzle in the stem block. A user inhaling through the drug delivery outlet of the device at this point will thus receive a dose of the drug.
Metered dose inhalers as described above administer an accurate dose of medicament whenever required, which is particularly useful for users whose respiratory difficulties manifest themselves suddenly. Such has been the success of these devices that they are now used throughout the world, where they are exposed to a wide variety of climatic conditions.
A more recent development is the so-called xe2x80x9cbreath-operated actuatorxe2x80x9d which delivers a dose of drug through a mouthpiece in response to inhalation by the user. This type of arrangement is particularly convenient in circumstances where the co-ordination between user inhalation and manual depression of the aerosol canister is imperfect. For example, children sometimes lack the necessary co-ordination to achieve effective self-administration. At times of respiratory distress, adult users may also experience poor co-ordination.
Unfortunately, one of the drawbacks of self-administration from an inhaler is that users often experience difficulty in determining when the charge in the medicament-containing vessel has nearly run out. With aerosol canisters, part of the reason for this difficulty is that a surplus of propellant may remain in the canister even though the drug supply is nearly used up. Alternatively, the near-exhausted state may result in a surplus of drug in relation to propellant. Thus, the illusion is created that the inhaler is still capable of providing useful doses of medicament simply because the canister contains liquid. This is potentially hazardous for the user since dosing becomes unreliable and because few people routinely carry a back-up device.
Many users have several different inhalers for the treatment of a variety of conditions. Others keep inhalers at a number of different locations such as at school, home, work etc. In these circumstances it is particularly difficult for the user to keep track of the amount of usage extracted from each individual inhaler apparatus.
Clearly there is a need for a counter mechanism which enables users to assess how many doses remain in the obscured canister. Such a counter would ensure that users are warned when the inhaler nears exhaustion so that appropriate measures can be taken to avoid running out of medication. Moreover, if a dose counter can provide readability to a resolution of one dose, this can be used for compliance monitoring, either under hospital supervision or by parents and teachers assessing compliance by children in their care.
To this end, various counters have been proposed in recent times which aid the management of metered dosage. Such counters vary in complexity and sophistication, but they all have in common the feature that they detect relative movement between the medicament-containing vessel and the actuator body and increment in response to such movement.
Known dose counters can be categorised into the following types:
(a) Reservoir low indicators,
(b) Fuel gauge types (resolution limited to 10 doses or higher),
(c) Dose computers (resolution to one dose and intrinsically accurate).
An example of the first type is described in International Patent Application No. WO 86/05991 which uses a wheel carrying a coloured mark as the warning indicator. The wheel is rotated by a worm drive forming part of the metering mechanism. Actuation of the metering mechanism infers that the patient has taken a dose of medicament and, when a predetermined number of doses has been dispensed, the coloured part of the wheel becomes visible through an aperture. This indicates to the user that a replacement dispenser will be required shortly. The principles disclosed in this document would enable the system to be upgraded to a fuel gauge type display except for the fact that the worm gearing system is unsuitable for providing single dose resolution in a portable product.
International Patent Application No. WO 92/09324 discloses the use of a rotatable display means incorporating a rack of teeth which is driven by a ratchet during the dispensing of a medicament dose. Each tooth on the rack corresponds to a single dose. The disadvantage of this type of arrangement is that, for reliable operation, all of the teeth in the rack need to be perfect. One poor tooth will result in the counter display showing xe2x80x9chalf fullxe2x80x9d when the reservoir is actually empty. The document also discloses a number of gearing means to enable the display to increment for hundreds of doses. To provide a counter for the typical 200-dose portable inhaler using this invention would require a wheel with very small teeth, formation of which is beyond current manufacturing capabilities at sensible costs.
European Patent Application No. 0 480 488 discloses a similar dose counter using a rack of single-direction teeth as the recording means. This arrangement typifies the current state of the art and its inherent limitations. The display ring shows the doses remaining, but only in units of ten. Furthermore, for reasons of tooth size, the display ring limits the reservoir capacity to a total of approximately counted 120 doses. By means of further reduction in gearing it is possible for this type of system to be extended to 200 doses but only by reducing the display resolution to greater than 10 doses.
U.S. Pat. No. 4,565,302 discloses similar means to the devices discussed above, but illustrated clearly in FIG. 6 the limitations of such mechanisms in terms of display readability and total reservoir size. To ensure readability, a large numeral 5 is shown in the device; it is clear from the Figure that no more than 30 doses could be displayed in this manner if the same style and size of indication is required for each dose dispensed i.e. 5, 4, 3 etc.
In U.S. Pat. No. 5,437,270 for a multi-dose powder device, a number of display means are disclosed, including a tape system. The system is described as a tape wound up into a roll and which is freely rotatable, the leading edge of the tape being secured to a drum coupled to the rotation of the metering member. The metering member is described as being rotated through a known angular increment by a ratchet system linked to a linear button that the patient presses to meter a dose.
An example of a dose computer is described in International Patent Application No. WO 91/06334. This document discloses an electronic method of counting doses remaining in the medicament canister using a switch triggered by the can movement: the switch actuation is recorded by a microprocessor which displays the remaining doses on a screen. This system was invented for monitoring clinical studies and includes date/time logging and means for data downloading. This was the first patent to show an implementation of a true electronic dose computer.
Numerous other patent applications have followed, disclosing other dose sensing means (usually inferred from the movement of the metering valve) coupled to electronic means of recording the doses.
One of the drawbacks of these known counters is that they rely on mechanical interaction between parts attached to the medicament-containing vessel and parts provided on the actuator body. In other words, they are displacement-triggered. Such counters are difficult to manufacture with satisfactory tolerances because of the variation in length of typical aerosol canisters which is attributable in part to the crimping operation used to connect the valve-carrying lid portion to the main cup portion. Another variable is the length of stroke of the metering valve. Although the technology involved is not especially demanding, it has been found that the amount of travel effective to actuate the metering valve of a typical medicament-containing aerosol canister may fall in a tolerance band as small as 0.5 mm. Thus it is difficult to provide a generic counter which increments accurately in response to every actuation. This may be true even when the counter, the aerosol canister and the inhaler housing have been specifically designed for use together. The problem is therefore likely to be worse in circumstances where different manufacturers"" aerosol canisters, inhaler housings and displacement-triggered counters are used in combination.
International Patent Application No. WO 96/00595 discloses the use of a heat sensor which detects the heat of evaporation of the propellent and thus truly detects the delivered dose. However, this device suffers from the common drawback of all electronic dose counters in that it is relatively expensive.
It is therefore an object of the present invention to combine the following advantages in a mechanical counter:
(1) Absolute intrinsic reliability in usexe2x80x94an unreliable counter is worse than no counter at all.
(2) Single dose readability over 200 doses or more.
(3) Low cost to suit disposable products.
(4) Small size with large display to suit portable inhalers.
In a first aspect, the invention is a dose counter for a metered dose inhaler, the counter comprising:
actuator means;
drive means for driving rotary gear means in step-wise fashion in response to displacement of said actuator means, said rotary gear means comprising a wheel mounted on a spindle and said wheel having a plurality of ratchet teeth around its periphery;
means to prevent reverse rotation of said rotary gear means;
display means coupled to the rotary motion of said rotary gear means, said display means having a visible array of incrementing integers on a surface thereof indexable by a single integer in response to each step of the step-wise rotary motion of the rotary gear means;
characterised in that said dose counter further comprises a control surface to regulate the position of engagement and disengagement between said drive means and said wheel.
Preferably, the actuator means are operable by linear displacement from a first position to a second position and back to said first position and the count index occurs during the forward stroke of the actuator means from said first position to said second position. For example, the actuator means may comprise a spring-loaded plunger adapted to engage the rim of a medicament reservoir and being depressible against the return force of the spring loading when the medicament reservoir is translated to deliver a dose of medicament through its metering valve. The actuator means may be integrally formed with the drive means.
By controlling the position of engagement and disengagement between the drive means and the wheel, the travel required to count is precisely regulated. For maximum reliability and accuracy, the counter must only index after the metering valve has delivered its dose from the inhaler. If it counts before this, users can index the counter without receiving a dose of medicament. In extreme cases, the user could end up with a counter reading empty when the medicament reservoir is actually full. Hence, the travel from rest to the fire point can be defined as the xe2x80x9cmust not countxe2x80x9d zone.
As soon as possible after firing, the counter needs to index, the upper limit being the extent of full travel of the metering valve. This can be defined as the xe2x80x9cmust countxe2x80x9d zone. Since all assemblies have tolerances necessary for reliable production, part of the xe2x80x9cmust countxe2x80x9d zone is required for the fire position variance and part for the full valve travel variance.
Although typical valve travels are between 3 and 5 mm, only part of this travel is available to index a counter mechanism. Hence the actual requirement for a counter is that it must index after 2 mm of travel and be capable of accepting 5 mm of travel without double counting.
IN known ratchet indexing mechanisms, a drive element is used to engage a one-direction tooth form of a rack. The rack may be linear or it may be turned upon itself to form a toothed wheel. The drive element engages in the first rack tooth and moves the whole rack a distance greater than one tooth pitch. The rack must then remain in the new position while the drive element disengages from the first tooth and engages the next. The temporary holding of the rack is commonly achieved by a fixed element or pawl that engages a tooth form on the rack and prevents reverse motion.
In order that only one tooth is indexed for each drive element movement, the rack travel must exceed one tooth pitch but be less then two tooth pitches. Thus, the chosen pitch must match the available travel, with nominal travel being set typically at 1.5 tooth pitches. The maximum range of travel is therefore between one and two tooth pitches.
Unfortunately the tolerances encountered in metering valves for inhaler devices fall outside this band. If the xe2x80x9cmust countxe2x80x9d zone begins at 2 mm of travel and extends to between 3 and 5 mm, it is impossible to determine a tooth pitch when is universally applicable to all inhaler variants.
The control surface overcomes this problem by regulating the point of engagement as well as the point of disengagement between the toothed wheel and the drive element. It then becomes possible to use a small circular rack to obtain precise increments. By extending the control surface beyond the circular rack in both directions, it is possible to provide a precise rotational increment from linear valve travel even when the linear motion far exceeds that required for the rotational increment. Using this invention, the maximum precision of incrementation is obtained without the possibility of double counting or sensitivity to variation in linear travel.
In an especially preferred variant, the means to prevent reverse rotation of the rotary gear means is a friction clutch such as a wrap-spring clutch. The advantage of a wrap-spring clutch is that it operates on the spindle mounting the ratchet wheel and braces the spindle against reverse rotation relative to the counter chassis. Absence of side forces acting on the ratchet wheel means that mechanical operation of the counter is simplified.
Advantageously, the drive means for driving the rotary gear means comprises a ratchet drive pawl in the form of a straddle drive in which the element that engages the ratchet teeth of the wheel is supported between a pair of spaced apart support arms. The gap between the support arms is dimensioned to accommodate the thickness of the wheel therebetween, so that the depth of engagement between the ratchet drive pawl and the ratchet teeth of the wheel is unhindered by the drive means support.
Preferably, the display means is an elongate flexible web such as a paper or plastics tape on which the dose amount is printed, say as a descending sequence of numbers from e.g. 200. The advantage of a tape display is that different print styles or representations can be easily incorporated to emphasise significant events to the inhaler user, such as approaching exhaustion of the medicament reservoir. The tape is dispensed from a supply spool which is arranged in parallel with a take-up spool. The take-up spool may share the same spindle as that on which the ratchet wheel is mounted. The tape extends between the two spools and passes behind a window in the inhaler apparatus through which one of the printed figures is visible to the user.
In a second aspect, the invention is a dose counter for a metered dose inhaler, the counter comprising:
actuator means;
drive means for driving rotary gear means in step-wise fashion in response to displacement of said actuator means, said rotary gear means comprising a wheel mounted on a spindle and said wheel having a plurality of ratchet teeth around its periphery;
display means coupled to the rotary motion of said rotary gear means, said display means having a visible array of incrementing integers on a surface thereof indexable by a single integer in response to each step of the step-wise rotary motion of the rotary gear means;
characterised in that said dose counter further comprises stepless restraint means to prevent reverse rotation of said rotary gear means.
Preferably, the stepless restraint means is a wrap-spring clutch operating on the spindle on which the ratchet wheel is mounted and bracing the spindle against reverse rotation relative to the counter chassis. Absence of side forces acting on the ratchet wheel means that mechanical operation of the counter is simplified.
The actuator means may be operable by linear displacement from a first position to a second position and back to said first position, the count index occurring during the forward stroke of the actuator means from said first position to said second position. For example, the actuator means may comprise a spring-loaded plunger adapted to engage the rim of a medicament reservoir and being depressible against the return force of the spring loading when the medicament reservoir is translated to deliver a dose of medicament through its metering valve. The actuator means may be integrally formed with the drive means.
The drive means for driving the rotary gear means may be a ratchet drive pawl in the form of a straddle drive in which the element that engages the ratchet teeth of the wheel is supported between a pair of spaced apart support arms. The gap between the support arms is preferably dimensioned to accommodate the thickness of the wheel therebetween, so that the depth of engagement between the ratchet drive pawl and the ratchet teeth of the wheel is unhindered by the drive means support.
Preferably, the display means is an elongate flexible web of paper or plastics material on which the dose count is printed, for example as a descending sequence of numbers from e.g. 200. The advantage of a tape display is that different print styles or representations can be easily incorporated to emphasise significant events to the inhaler user, such as approaching exhaustion of the medicament reservoir. The tape is dispensed from a supply stool which is arranged in parallel with a take-up spool. The take-up spool may share the same spindle as that on which the ratchet wheel is mounted. The tape extends between the two spools and passes behind a window in the inhaler apparatus through which one of the printed figures is visible to the user.
In a third aspect, the invention is a dose counter for a metered dose inhaler, the counter comprising:
actuator means;
drive means for driving rotary gear means in step-wise fashion in response to displacement of said actuator means, said rotary gear means comprising a wheel mounted on a spindle and said wheel having a plurality of ratchet teeth around its periphery;
display means coupled to the rotary motion of said rotary gear means, said display means having a visible array of incrementing integers on a surface thereof indexable by a single integer in response to each step of the step-wise rotary motion of the rotary gear means;
characterised in that said dose counter further comprises a ratchet and drive pawl mechanism including a rack having an array of ratchet teeth and drive pawl means in the form of a straddle drive in which the element that engages the ratchet teeth is supported between a pair of spaced apart support arms.
The gap between the support arms is preferably dimensioned to accommodate the thickness of the rack therebetween, so that the depth of engagement between the ratchet drive pawl and the teeth of the rack is unhindered by the drive means support. This arrangement helps to equalise the forces on either side of the tooth-engaging element and minimises twisting. The drive pawl may be used either to pull the ratchet teeth or to push them.